1. Field of the Invention
This invention relates to a radiation image readout apparatus which detects the light emitted by a stimulable phosphor sheet on which a radiation image is recorded to therby obtain an image signal representing the radiation image. It further relates to an erasing apparatus for erasing the radiation energy remaining on the stimulable phosphor sheet after read-out of the radiation image has been completed.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in Japanese Unexamined Patent Publication No. 56(1981)-11395, and U.S. Pat. Nos. 4,258,264, 4,315,318, 4,387,428 and 4,276,473, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a very wide range in proportion to the amount of energy stored thereon, it is possible to obtain an image which is unaffected by variations in the amount of exposure of the stimulable phosphor to the radiation, even when the amount of exposure varies greatly depending on the imaging conditions, by reading out the emitted light with a photoelectric conversion means at an appropriate read-out gain and converting it to an electric image signal to reproduce a visible image on a recording medium such as photographic film or on a display device such as a CRT.
In such systems, there are sometimes employed different types of stimulable phosphor sheets and, as a result, two sheets exposed to the same amount of radiation energy at the time of image recording may be found to emit different amounts of light during read-out even when scanned with stimulating rays under the same conditions. Thus where no particular precautions are taken regarding the read-out of different types of stimulable phosphor sheets and read-out is conducted under identical conditions for all of the sheets regardless of type, it is likely that overflow of the image signal will make it impossible to obtain an image signal carrying accurate radiation image information. The density of the visible image reproduced using such an image signal is apt to be either too high or too low. (This also affects the accuracy of the reproduced image in the case where it is displayed on a CRT or the like.) The reproduced image is thus troublesome to examine.
The radiation image read-out in the aforesaid manner generally does not release all of the radiation image energy stored by the stimulable phosphor sheet and the sheet usually continues to carry a residual image after completion of read-out. For restoring the stimulable phosphor sheet to a usable state, therefore, the read-out sheet is exposed to erasing light for discharging the remaining radiation energy (erasing the residual image).
There is a high probability that a given system will use different types of stimulable phosphor sheets with different degrees of erasability. Erasability is expressed in terms of the amount of erasing light required to erase the residual image of a stimulable phosphor sheet that has been exposed to a specified amount of radiation energy and thereafter read out under specified conditions. Stimulable phosphor sheets having different erasabilities require different amounts of residual image erasing light even when they are exposed to the same amount of radiation energy and read-out under the same conditions.
One solution to this problem would be to set the amount of erasing light to that appropriate for the type of stimulable phosphor sheet requiring the largest amount of erasing light (where the amount of erasing light equals the product of the amount of light emitted by the erasing light source per unit time and the period of time that the stimulable phosphor sheet is exposed to the erasing light). Since some types of stimulable phosphor sheets would be completely erased by exposure to a smaller amount of erasing light, however, this method would frequently waste electric power by operating the erasing light source at a higher output or for a longer time than necessary. It would also prolong the operation cycle time in a system that circulates one or more stimulable phosphor sheets for repeated use.